1. Field of the Invention
The present invention relates to an imaging lens, and in particular to an imaging lens suitable for mounting in a portable telephone, image input device for a personal computer, digital camera, surveillance CCD camera, inspection device, or similar equipment, which uses a CCD or CMOS device as an imaging component.
2. Description of Related Art
In the above-described imaging lens, it is necessary that the optical length, defined as the distance from the incidence plane on the object side of the imaging lens to the image plane (image-forming surface of the CCD or similar), be short. That is, in the design of the imaging lens, measures must be taken to reduce insofar as possible the ratio of the optical length to the combined focal length of the imaging lens. Hereafter, an imaging lens with a short optical length, that is, the ratio of the optical length to the focal length of which is small, may be called a “compact” lens.
Taking a portable telephone as an example, the optical length must at least be made shorter than the thickness of the portable telephone unit. On the other hand, it is preferable that the back focus, defined as the distance from the emission plane on the image side of the imaging lens to the image plane, be as long as possible. That is, in the design of the imaging lens, measures should be taken to increase insofar as possible the ratio of the back focus to the combined focal length of the imaging lens. This is due to the need to insert filters, cover glass, and other components between the imaging lens and the image plane.
In addition to the above, it is of course required that the various aberrations of an imaging lens are corrected to amounts sufficiently small that distortion of the image formed by this imaging lens is not recognized by visual perception, and sufficiently small as to satisfy the requirements of the integration density of the imaging components (also called “pixels”). That is, the various aberrations must be corrected satisfactorily; below, an image for which various aberrations are satisfactorily corrected may be called a “satisfactory image”.
As stated below, imaging lenses with a two-component configuration have been disclosed which are appropriate for use in image equipment employing a CCD, CMOS device or other solid-state image pickup component, of which portable computers and videophones are representative. In addition to obtaining satisfactory images, such lenses are all designed for small size and light weight.
Of these, as a first lens, an infrared lens has been disclosed with a two-component configuration comprising two meniscus lenses (a first lens and a second lens), designed for low cost and for reduced weight, which is compact and has adequate image-forming performance for practical use. (See for example Japanese Unexamined Patent Application Publication No. 2000-75203.)
However, such an infrared lens has a broad interval D between the first lens and second lens, so that as a result the optical length is necessarily long, and design of a sufficiently compact lens system is difficult. The ratio D/f of the interval D between the first and second lenses to the focal length f of the entire system (the combined focal length of the two-component lens system) is at least 0.8. Consequently, the ratio of the optical length to the focal length of the entire system is large at approximately 1.5 (the value for the lens in Embodiment 6, with the smallest value, is 1.4236), so that the optical length of such lenses is long.
As a second two-component lens, an anamorphic attachment lens has been disclosed (see for example Japanese Unexamined Patent Application Publication No. 2000-81568), in which an image of the object is focused with different vertical and horizontal magnifications by one afocal lens comprising cylindrical surfaces both of which have refractive power only in the same direction and have radii of curvature with the same sign, and which can project images more slim or more thick than the actual object. Such an anamorphic attachment lens is positioned and used on the object side of a still camera lens system in particular.
However, while the anamorphic attachment lens is a lens system configured as a two-component lens, the surface shapes of the component lenses of the anamorphic attachment lens are not spherical, as in the case of ordinary lenses, but are cylindrical surfaces. Hence the basic configuration differs from that of the imaging lens of the present invention.
As a third two-component lens, an objective lens for recording and reproduction of information on optical information recording media a large operating distance for which is secured has been disclosed, in which a lens group having two components with positive refractive power has a large NA (numerical aperture) of 0.85, and which is appropriate for incorporation into optical pickup devices with light source wavelengths of 500 nm or below, with chromatic aberration corrected satisfactorily (see for example Japanese Unexamined Patent Application Publication No. 2003-167187). And, as fourth through sixth two-component lenses, objective lenses for high-performance, small-size optical pickups, having a large numerical aperture and long working distance, have been disclosed (see for example Japanese Unexamined Patent Application Publication No. 2003-5026, Japanese Unexamined Patent Application Publication No. 2003-5027, and Japanese Unexamined Patent Application Publication No. 2003-5055).
However, these objective lenses are designed with a small NA, for the purpose of using the objective lens to focus parallel rays into as small an area as possible on the information recording surface; the imaging lenses of the present invention are inherently different in concept from these. Consequently the values of the focal lengths, back focuses, and optical lengths of each of the two lenses making up the lens system, and the interval between lenses, are different from the imaging lenses of the present invention. Whereas a numerical aperture of 0.85 or higher, converted into an F-number, yields a value of 1 or below, the F-number of an imaging lens of the present invention (Fno) is in the range 2.0<Fno<4.0, for a very bright lens.
As indexes representing the brightness of a lens (or lens system), in order to distinguish between the numerical aperture (NA) represented by the product n·sin(u) of the object-side refractive index n and the angle u subtended by the radius of the entrance pupil, and the F-number represented as the ratio f/Din of the lens focal length f to the diameter of the entrance pupil Din, in this specification a value representing the numerical aperture will be denoted by NA, and a value representing the F-number will be denoted by Fno. These two variables are, in approximation, reciprocals of each other.
As the seventh two-component lens, a bright, compact infrared lens is disclosed (see for example Japanese Unexamined Patent Application Publication No. 2003-295052), in which comparatively inexpensive material is used, costs are reduced, and the effects of unwanted orders of diffracted light, which are a disadvantage of diffraction gratings, are eliminated. Of the two component lenses of this infrared lens, a diffraction grating is formed on the concave surface of a meniscus lens positioned on the object side. Hence the components are completely different, and the lens system completely different, from an imaging lens of the present invention, which does not employ a diffraction grating as a component.
As the eighth two-component lens, an image-forming lens is disclosed comprising a front-group lens having positive refractive power, an aperture, and a rear-group lens, with Fno=4, a half-image angle of approximately 24°, with aberrations satisfactorily corrected, and appropriate for image pickup in fax equipment and similar (see for example Japanese Unexamined Patent Application Publication No. 7-181379).
However, this image-forming lens system has a short back focus, so that insertion of a filter or similar between lenses and image plane to block infrared light is difficult. The ratio bf/f of the back focus bf to the focal length f of the entire system (the combined focal length of the two-component lens system) is at most 0.38 (Embodiment 5).
As the ninth through 11th two-component lenses, imaging lenses are disclosed comprising an image-forming lens (first lens) and corrective lens (second lens) (see for example Japanese Unexamined Patent Application Publication No. 2000-66094, Japanese Unexamined Patent Application Publication No. 2000-66095, and Japanese Unexamined Patent Application Publication No. 2000-66096).
However, these imaging lenses have a configuration in which the thickness of the image-forming lens on the optical axis is large. Consequently the refractive index tends to be distributed unevenly when configuring the lens, and image distortion and similar occur due to such uneven distribution of the refractive index, so that in some cases the image quality which should result from lens design cannot be obtained.
As the 12th two-component lens, an imaging lens with comparatively small image dimensions, and which in particular enables performance appropriate to mounting in small-size image pickup equipment, is disclosed (see for example Japanese Unexamined Patent Application Publication No. 2004-4620). This imaging lens is configured by positioning, in order from the object side, an aperture diaphragm, a first lens, and a second lens. That is, a configuration is employed in which a diaphragm is not provided between the first and second lens. Consequently flare cannot be eliminated adequately, and consequently there are limits to the ability to raise the contrast of the image and obtain sharp images.
Further, the ratio d/f of the optical length d to the focal length f for the entire system (the combined focal length of the two-component lens system) exceeds 1.3, except for the imaging lens disclosed as Embodiment 9. That is, in this design the optical length is long, and compactness cannot be completely attained. On the other hand, the ratio d/f of the optical length d to the focal length f for the entire system of the imaging lens disclosed as Embodiment 9 is 1.137; but the ratio D2/f of the interval D2 between the first and second lenses to the focal length f of the entire system for this imaging lens is extremely small, at 0.11. Consequently in this configuration, a diaphragm could not easily be inserted as a second diaphragm between the first and second lenses.
However, in order to accommodate the more compact designs of portable telephone units, image input equipment for personal computers and other equipment, it is desirable that the optical length of the imaging lens mounted in such equipment be short, and in addition it is required that satisfactory images be captured.
An object of the present invention is to provide an imaging lens, the F-number of which is in the range 2.0 to 4.0, configured with only two component lenses, with a short lens optical length, and capable of obtaining satisfactory images. In addition, a further object is to provide an imaging lens in which, by providing a diaphragm between the first lens and second lens to adequately eliminate flare, image contrast can be increased sufficiently that sharp images can be obtained.
A further object is to be provide an imaging lens which achieves low costs and light weight by using plastic material for all the lenses (two lenses) making up the imaging lens. Here, a “plastic material” is a polymer material which can be molded to form a lens by plastic deformation using heat or pressure, or both, and which is transparent to visible light.